Method and apparatus for drilling well bores



April 21, 1970 J. D. BENNETT ETAL METHOD AND APPARATUS FOR DRILLING WELL BORES Filed July 9, 1968 MUD PUMP

PROGRAMED VALVE AIR PUMP ACCUMULATOR ACCUMULATCJR FROM ANNULUS FIG. 2B

uoum a souo SHALE SHAKER s R TN Y. 0 O E T SEH N N N R wmw m N BJ D MW ND f HREH oo o JFFJ l 3 m F F United States Patent 3,507,344 METHOD AND APPARATUS FOR DRILLING WELL BORES John D. Bennett, Richardson, Tex., Ford L. Johnson, Fallbrook, Calif., and Fred M. Mayes, Richardson, and John W. Peret, Dallas, Tex., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Filed July 9, 1968, Ser. No. 743,427 Int. Cl. E21b 21/04 U.S. Cl. 175-69 12 Claims ABSTRACT OF THE DISCLOSURE The particular embodiment described herein as illus trative of one form of the invention utilizes, in a drilling operation, a system for introducing alternate slugs of liquid and gas into the well bore as the drilling fluid to increase the drilling rate.

BACKGROUND OF THE INVENTION This invention relates generally to methods and apparatus for drilling well bores, and more particularly to a drilling system for increasing the drilling rate by introducing slugs of a gas into the drilling fluid.

In order to point out the advances represented by the invention disclosed herein, it will be helpful first to consider briefly, certain aspects of standard drilling practices. In rotary drilling, a drill bit is carried or rotated by a hollow drill stem, through which a heavy drilling fluid is pumped, which is usually termed mud. The mud emerges from the drill stem through the bit to lubricate the cutters on the bit in their cutting action, and to flush away the cuttings which are then brought to the surface by the return column of mud surrounding the drill stem. The mud penetrates exposed interstices, or crevices of the formation, through which the cut is being made. This intrusion of mud which is in the form of a mud cake about the borehole wall, serves to hold formation pressure in check, thereby preventing inflow of gas or oil, and also preventing the walls of the bore from caving or breaking down. Such mudding off of the bore during drilling operations maintains the well in a dead condition throughout the drilling operation, with the mud cake and circulating pressure of the mud preventing the inflow of formation gas, oil, or other formation fluids. Normally the weight or gravity of the mud is maintained at a level such that the hydrostatic head produced by the column of mud in the well bore will be sufficient to maintain a mud cake form about the borehole wall, but also at the bottom of the borehole which is being drilled by the drill bit. As the bit grinds into the earth formation, the mud is continuously being applied to the bottom of the hole, so that a positive back pressure is maintained against the formation being drilled at all times. Such a positive pressure on the formation at the bottom of the borehole is a direct deterrent to the removal and lifting of cuttings from the borehole, with the pressure tending to hold the cuttings down rather than permitting their removal to the surface with the mud column. If such mud pressure can be reduced on the formation at the floor of the borehole, the internal formation pressures themselves will then provide a positive expelling force for the portions of the formation being cut by the drill bit. Such an action would substantially increase the drilling rate, and thereby effect economies in the drilling operation.

In addition, studies of rock failures under a bit show that with a low positive differential pressure on the for- 3,507,344 Patented Apr. 21, 1970 Mice mation, the rock fails in a brittle manner, and a large volume of rock is removed from under the bit teeth. Under high differential pressure, however, the rock fails in a psuedo plastic manner, and a much smaller volume of rock is removed. Studies of laboratory drilling rates on a quantitative basis in comparison to diiferental pressures show that at a high differential pressure, 3,000 or 4,000 p.s.i. over balance, even a rather large change in differential pressure produces only a very small change in laboratory drilling rate, or crater volume under the bit tooth. However, as the differential approaches zero, from about 500 p.s.i. over balance on down to the balance point, the drilling rate increases 3 or 4 fold, and this of course is very significant. Field studies also confirm these relationships which have been found in laboratory research. It has been found that at very high over-balance, there is very little change in drilling rate, with changes in differential pressure until the diiferential reaches the 500 p.s.i. point and approaches the actual balance point. In this interval, a very drastic increase in drilling rate takes place, with further decreases in diiferential pressure.

Recent advancements in pressure detection technology have led to more precise techniques for the detection of formation pressures, so that pressure balances may be more closely controlled. Such detection techniques would also be applicable to the present invention as a means for maintaining proper pressure controls in the well bore when using the gas slugs as a drilling aid.

It is therefore an object of the present invention to provide a new and improved technique for drilling well bores.

SUMMARY OF THE INVENTION With this and other objects in view, the present invention contemplates a method of drilling into earth formations while utilizing a drilling fluid comprised of alternate slugs of a liquid and a gaseous material, which are introduced into the well bore during the drilling operation.

Apparatus is provided for accumulating volumes of the liquid and gas at the surface, with automatic means for introducing the slugs into the drill stem, and in addition, surface equipment is provided for receiving the mud with gas slugs therein and for separating the gas from the mud system.

A complete understanding of this invention may be had by reference to the following detailed description, when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic view of a well bore, including a drill stem and bit utilized in a rotary drilling operation, and illustrating the principles of the present invention;

FIGURES 2A and 2B are schematic illustrations of surface equipment utilized in the system of the present invention; and

FIGURE 3 is a partial schematic view of the well bore illustrating the drilling operations during a liquid phase of mud circulation about the bit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGURE 1 of the drawings, a drill stem 12 is shown extending downwardly within a well 'bore 14. The drill stem 12 is comprised of sections of drill pipe, and includes a bit 16 positioned at its lower end. The bit includes a rotating member having teeth 1-8 thereon, for removing portions of the formation to accomplish the drilling operation. The drill pipe 12 has a hollow interior bore 20 which communicates with an interior bore 22 within the drill bit. A passage 24 communicates the bore 22 in the bit with the exterior of the bit along its lower side. The bores within the drill stern and bit, including the communicating passage 24, are for the purpose of passing a drilling fluid from the surface through the drill stern and out the lower end of the bit during the drilling operation. Such circulation of fluid provides a means for lubricating the bit, carrying removed portions of the formation to the surface, and providing a hydrostatic back pressure against formation pressures.

Various drilling fluid systems are utilized in order to maintain proper pressure control within the well here. For example, if high bottom-hole pressures are expected to be encountered, heavy drilling fluids are used which may be two or more times as dense as water. Water also has been used as a drilling fluid in order to maintain a low pressure against the formation, which in turn normally increases the rate of drilling. Lighter than water fluids, such as oil, are also used in drilling operations as well as aerated fluids which tend to further decrease the weight of the drilling fluid. None of the above described systems however, provide the advantages of the present system, which includes, as shown in FIGURE 2, a means for introducing alternate slugs of a gas and liquid through the drill stem for purposes to be hereinafter described.

As shown schematically in FIGURE 2A, the surface equipment includes a mud pump 30, which is connected by an appropriate piping system to a mud accumulator 32. The accumulator discharges into a programmed valve 34. An air pump 36 is connected by piping to an air accumulator 38 which in turn is also communicated with the programmed valve 34. The valve 34 has a discharge line or pipe 4! which connects with the drill stem by means of conventional drilling fluid system apparatus.

In the operation of the apparatus thus far described, the mud and air pumps 30 and 36 respectively are operated continuously to maintain a supply of liquid and gas to the respective accumulators 32 and 38, which in turn feed to the programmed valve 34. The valve is operated to connect alternate flow channels between the pipe 40 and communicating lines with the accumulators 32, 38. The programmed valve is operated in a timed sequence so that a desired volume of liquid or gas is alternately supplied to the pipe 40, which in turn communicates with the interior of the drill pipe through a conventional swivel arrangement schematically represented at 42. Drive means (not shown) would of course be supplied, for example, if a rotary drilling operation were used, for rotating the drill pipe and bit.

As shown in FIGURE 1, alternate slugs of mud 44 and air 46, after being introduced into the drill pipe 12, progress downwardly within the interior bore of the pipe. The air slug is compressed under the influence of the increasing column of weight on the air slug as it progresses downwardly within the pipe. Therefore, it is necessary to introduce a very large volume of air into the drill pipe at the surface in order to provide a sufficient volume of air or gas at the bottom of the well bore for performing the invention described herein. For example, if a 12 pound mud or drilling fluid were used in the drilling operation for purposes of balancing pressure forces within the formations traversed by the well bore, a gas slug, at a position 10,000 feet below the surface would be subjected to approximately 400 atmospheres. This approximation does not, however, take into effect the lessened weight of the mud column due to other air slugs being entrained therein, or temperature considerations which would increase with the depth of the well, and therefore would tend to increase the volume of gas somewhat at the bottom of the well bore. In any event, it is readily appreciated that a great volume of gas, for example 200 to 400 times as much gas, would be introduced into the drill pipe at the surface, than would be required at the bottom of the well bore for performing the method according to the present invention.

As the alternate slugs of liquid and gas reach the bottom of a well bore, and provide such alternate environments in which the bit operates, the conditions as suggested in FIGU RES 1 and 3 would exist. First referring to FIGURE 1, when the air slug reaches the bottom of the drill stem and emerges into the well bore to provide a gas environment about the bit, the mud cake (represented by the darkened area in the well bore) which is normally present on the bottom of the well bore is removed by the rotating drill bit. This permits the gas to penetrate into the formation below the bit. In addition, the high velocity gas emitting through the passage 24 tends to also help remove the mud cake from the bottom of the bore. Penetration of the gas phase into the formation permits a pressure equalization or positive pressure under the chips or formation fragments to reduce the chip hold-down forces normally encountered when drilling in the liquid phase. It is pointed out that the gas phase of the drilling fluid might be carbon dioxide or engine exhaust gas to decrease the danger of an explosion in the mud system, as for example, if ignitable formation gas were encountered during the drilling operation.

As shown diagrammatically by the arrows in FIGURE 1, the gas phase penetrates into the formation below the bit to provide such equalizing pressures above and below portions of the formation being removed by the bit. This in itself provides for a higher drilling rate in that it tends to eliminate the hold-down pressure, which is normally exerted by a positive differential pressure in the lower end of the well bore. Elimination of the hold-down pressure permits the formation fragments to be readily lifted by the circulating drilling fluid for removal to the surfaoe. It is expected that during the gas phase, the pressure around the bit is somewhat higher than during the liquid phase due to the rapid expansion of a gas through the bit passage 24 which causes a surge of pressure greater than that exerted by the drilling fluid column in the well annulus.

Referring next to FIGURE 3, it is seen that when the liquid phase of the drilling fluid occupies the lower end of the well bore, a mud cake is again formed on the bottom of the bore. However, the higher pressure gas (represented by the arrows in FIGURE 3) which has penetrated the formation, tends to move upwardly through the cake toward the lower pressure environment of the liquid phase to provide a negative differential pressure in the well bore which tends to move the chips and formation fragments upwardly from the bottom of the well bore. Even if no differential pressure were to exist across the mud cake after the liquid phase had entered the lower end of the bore, a mere equalization of pressure across the mud cake would provide for an increased drilling rate in that the formation fragments beneath the bit would not be subjected to the positive hold-down pressure which exists with a conventional single-phase drilling fluid system. It is seen that the fluctuation of drilling fluid pressure, which occurs as alternate slugs of gas and liquid occupy the lower end of the well bore, will cause cyclical surging of the formation face at the bottom of the well bore. Such dynamic forces will benefit the removal of formation fragments and increase the rate of drilling.

Upon return of the drilling fluid to the surface through the annular space between the drill pipe and the wellbore, the compressed slugs of gas will tend to expand back to the original volume which they occupied upon introduction into the drill pipe. Although there may be considerable intermixing of the liquid and gas phases during the return trip of the drilling fluid to the surface, it is likely that there will still be some slugs of air present in the drilling fluid when it reaches the surface, and therefore, certain precautions must be taken to provide control measures in the system which will prevent uncontrolled flow of drilling fluids at the surface. For example, the blow-out preventers, which would normally be open during a drilling operation except to provide safety precautions would, in the system disclosed herein, probably be maintained in a closed condition with suitable sealing means 49 provided between the upper closure on the well and the rotating drilling pipe to provide a sealed upper end to the well bore. A flow line is connected to the surface casing to permit the exit of drilling fluids into a fluid recovery system at the surface.

Such a recovery system is shown schematically in FIG- URE 2B of the drawings. The flow line 50 connects with the upper end of the surface casing to provide a flow path for drilling fluids exiting from the well bore. A valve 52, in the flow line, provides a means for controlling back pressure on the mud circulating system. The flow line empties into a separator 53 for removing the gaseous portions of the drilling fluid and permitting the liquid and solid portions to escape from the separator. The separator shown in FIGURE 2B is in the form of a cyclone type separator which provides for the entry of the drilling fluid at a tangent to the walls of a cylindrical vessel 54. This causes a swirling of the entering fluid about the walls of the vessel. Any gas in the system tends to escape through an upper gas escape pipe 56, while the liquid and solid components of the fluid tend to gravitate to the lower end of the vessel 54, and escape therefrom through a cone-shaped exit 58. Such escaping materials would then pass over a shale shaker or a similar apparatus 60, for removing the solid particles or formation chips from the drilling fluid. The liquid portions of the fluid then pass into a recovery vessel or pit (not shown) for recirculation in the drilling fluid system.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A method for drilling a well bore into earth formations comprising the steps of: operating a drilling member in the earth formation for removing portions from the earth formation; and passing a drilling liquid and a gas in alternate slugs through said drilling member while continuing the operation of such drilling member.

2. A method for drilling a well bore into earth formations comprising the steps of: operating a drilling member in the earth formation for removing portions from the earth formation; and subjecting the area about such drilling member to alternate slugs of liquid and gas while continuing the operation of such drilling member.

3. A method for drilling earth formations comprising the steps of: operating a rotating bit within a bore in earth formations to remove portions thereof; and passing a liquid and gas in alternate slugs into the bore about such bit for removing such portions to the surface.

4. The method of claim 3 and further including the step of separating gas from such portions upon their removed to the surface.

5. A method for drilling a well bore into earth formations comprising the steps of: rotating a drill bit suspended from drill pipe in a bore for removing portions from an earth formation; and passing alternate slugs of a drilling liquid and a gas into such bore for transporting such removed portions of earth formation to the surface.

6. The method of claim 5 wherein said drilling liquid is comprised of materials having a density greater than that of water.

7. The method of claim 5 wherein said gas is air.

8. In a rotary drilling operation, an improved method of drilling into earth formations which comprises: rotating drill pipe and a bit thereon to drill a borehole into earth formations; passing a drilling fluid through such drill pipe and bit to the lower end of the borehole and then to the surface on the outside of the pipe to remove formation cuttings to the surface and to maintain back pressure on the formations transversed by such drilling operation, and; alternating the drilling fluid constituents between a liquid and gas to provide alternate slugs of liquid and gas about the bit while continuing rotation of the bit in the borehole.

9. The method of claim 8 and further including the step of separating gas constituents from the drilling fluid upon return thereof to the surface.

10. An apparatus for drilling a well bore in earth like formations comprising: boring means engaging the earth like formations; fluid circulating means connected to said boring means for passing drilling fluids into the well bore to transport separated portions of the earth like formations; and means for introducing alternate slugs of gas and liquid into such fluid circulating means, including accumulator means, employed such that the environment surrounding said boring means will substantially alternate between liquid and gas, both said accumulator means and slug introducing means being located at the surface and connected in series with said fluid circulating means.

11. The apparatus of claim 10 and further including means at the surface for separating gas from the liquid constituents of the drilling fluid and from the removed portions of earth formation.

12. The apparatus of claim 10 wherein said fluid circulating means includes a gas and liquid pump means located to introduce drilling fluids to the well bore, and further including automatically operated valve means connected to said pump means for alternately introducing the gas and liquid slugs into the well bore.

References Cited UNITED STATES PATENTS 1,867,832 7/1932 Hill 69 2,880,965 4/1959 Bobo 17569 2,951,680 9/1960 Camp 17569 3,107,738 10/1963 Osborn 175--69 X 3,268,017 8/1966 Yarbrough 175-69 X 3,387,672 6/1968 Cook 175-69 NILE C. BYERS, 111., Primary Examiner 

